PURPOSE: Observers report that a filled-in color from a chromatic light into an equiluminant achromatic surround is bounded by illusory contours (Feitosa-Santana et al, VSS 2009), but a possible explanation is that observers report filling-in because they cannot accurately localize illusory contours. This was tested by measuring (1) observers' ability to localize illusory contours and (2) the frequency of perceived filling-in when the chromatic light that normally fills-in had a higher luminance than the surround. If contour lozalization is poor, then frequency of filling-in should not vary with luminance because the added luminance-contrast edge still reaches a poorly localized illusory contour. METHODS: Three kinds of illusory contours were tested: Kanizsa square from solid “pacmen”, Kanizsa square from “bull's eye” pacmen, and horizontally phase-shifted vertical lines. In experiment (1), two thin dark horizontal lines on an achromatic background were presented on either side of a horizontal illusory contour. In different trials, the lines were positioned at various positions above or below the illusory contour; observers indicated whether the lines appeared above or below the contour. In experiment (2), a yellow square with a luminance higher than its achromatic surround was presented some distance from the illusory contour. Without luminance contrast, the yellow square fills-in up to the contour, which was either 4 or 6 min away. Three levels of luminance contrast were tested (5%, 7%, 11%). Observers indicated whether the yellow square appeared to be touching the illusory contour (thus a filled-in color). RESULTS & CONCLUSION: (1) Observers perceived the illusory contour's position with accuracy ±1min. (2) The frequency of filling-in was attenuated with 5% or 7% luminance contrast, and abolished at 11%. Both results are inconsistent with poorly localized illusory contours, and thus confirm that the spread of filled-in color is bounded by illusory contours.